1. Field of Invention
The present invention relates to a method for compensating pixel image non-uniformity; particularly, it relates to such a method capable of compensating non-uniformity of a pixel image by capacitance trimming, and an image sensor having pixels with variable capacitors for non-uniformity compensation.
2. Description of Related Art
An image sensor captures an image and converts it to electronic signals. FIG. 1 shows a conventional image sensor 10 which includes an array of pixels 11 arranged by columns and rows. Such an image sensor for example can be used in an optical mouse, a digital camera, etc. In such a conventional image sensor, it often occurs that an image is not uniformly received by every pixel in the array. This could be due to the non-uniformity of the pixels themselves or due to the optical system mated with the sensor, or a combination of both. In the case of non-uniformity due to the optical system, this is usually due to the non-uniformity of the imaging lens system (also referred to as the “lens roll off” or “relative illumination” or “vignetting” of the lens) which is often provided in front of the image sensor, or the non-uniformity of the illumination system, or both, which causes a deviation of the brightness across the array. For example, the brightness of pixels near the edge of the image may be lower than the brightness of the pixels near the center of the image, although the original brightness of the object maybe the same or uniform. As a result, the pixels near the edge of the array suffer brightness degradation and is considerably darker than the pixels at the center.
This non-uniform brightness constitutes noise or a fixed pattern(s) to the system, and it remains relatively static even as the captured image changes. This noise or fixed pattern may undesirably affect the tracking performance of an optical mouse such as accuracy, maximum speed achieved or surface coverage, and also may undesirably affect the performance of a digital camera or other devices using the image sensor.
In view of the above, to overcome the drawback in prior art, the present invention proposes a method for compensating for the non-uniformity of the pixel image and an image sensor having pixels with variable capacitors for non-uniformity compensation. Such a method and image sensor is capable of compensating non-uniformity of a pixel image by capacitor trimming.